Checking Null values and Not null values in foxpro9 - visual-foxpro

Question

I got some fields from SQL server into my Foxpro cursor as below.
stra="SELECT cCm_Sgl_TaxInv,cInvNo FROM MIS.dbo.vInvFinalAll where cInvNo=?thisform.txtInvoiceNo.value"
SQLEXEC(hndOps,stra,'TaxInv')
And I need to check null values and not null values using IF condition. I wrote the following code and it does not give the expected result.
SELECT TaxInv
IF NOT ISNULL(cCm_Sgl_TaxInv)
ELSE
ISNULL(cCm_Sgl_TaxInv)
thisform.cmdCreate.Enabled = .T.
MESSAGEBOX("The value is already inserted")
endif
How can I do this?

Answer 1

It is not very clear what you are asking here, checking your code, probably this is what you meant:
IF ISNULL(TaxInv.cCm_Sgl_TaxInv)
thisform.cmdCreate.Enabled = .T.
MESSAGEBOX("The value is already inserted")
endif
EDIT: This could be simpler for what you are trying to do:
text to m.stra noshow
SELECT case when cCm_Sgl_TaxInv is null then 1 else 0 end as txStatus
FROM MIS.dbo.vInvFinalAll
where cInvNo=?thisform.txtInvoiceNo.value
endtext
SQLEXEC(m.hndOps,m.stra,'TaxInv')
if (TaxInv.txStatus = 1)
thisform.cmdCreate.Enabled = .T.
MESSAGEBOX("The value is already inserted")
endif

Answer 2

You only seem to need to do something if the value is null, therefore assuming that:
The 'taxinv' cursor will only have one record.
You want to enable the command button only if the value is null
select taxinv
if isnull(taxinv.cCm_Sgl_TaxInv)
thisform.cmdCreate.Enabled = .t.
else
messagebox("The value is already inserted.")
endif